Modulator



Feb. 17, 1942. c. l.. wExs, JR 2,273,543

' MoDULAToR Original Filed Feb. 3, 1940 3 Sheets-Sheet 1 c. 1... WE/sJ/e.

BVN

Feb.'17, 1942. c. L. wels, JR Y 2,273,548

MODULATOR Original Filed Feb. '3, 1940 3 Sheets-Sheet 2 REC- zszokc. N0 an.' 55 .5.5.5 05u00.

/A/l/E/V TOR C. L. WE/S, JR.

By W

THQ/Wil Feb. 17, 1942. c. l..l wals, JR 2,273,548

MODULATOR v Original Filed Feb. 5, 1940 3 Sheets-Sheet 3 A TQELVQ..

{U-Nirso STAT v ,Patentedfeh 17, v1942 d -MoDULA'roR Y Charles L. Weis, Jr., Mount VernomN. Y.as Vsignor to Bell Telephone Laboratories, Incorporated,New York, N

' York Y., a corporation of New Original application February 3, 1940, Serial No.

Divided and this'application 4Septem- This invention relates .to electric signaling and signals are transmittedV 'in the region immedimoreparticularly to `modulators adapted for use -in electro-optical image production.

e This application is a division of copending applicationSerial No. 317,104, filed "February 3, 1940. Y e An object of the invention claimedy in this divisional -application is to provide an improved i modulatorof the`kind in which tWo electric discharge tubes areemployed each including a cathode and a plurality ofr other electrodes, on one of--which tubesalone the modulating cur-- rents are impressed and on both of which they carrier waves are impressed, the connections beingl such that the unmodulated component ofY carrier currentvis suppressed in the work circuit.

Other objects and features of the invention Y ately below 100 kilocycles andthe synchronizing *frequencies just below the range assigned to will be apparent as the description o'fthe inven" tion proceeds. v Amodulator embodying the invention' of lthis divisional application has been found to functem wherein the videosignals extended from zero` vfrequency upward. In such a system, herein `chosen for purposes of illustration and later det scribed in detail, a coaxial cable is employed as the transmission, medium for simultaneously" sound frequencies. For the transmission of `tele- I vision on the basis of 24 frames per second and a resolution of 240 lines, Vwhich is here chosen by way of example, the frequency band resulting from the scanning operation extends from zero to vabout 806 ,kilocycles. The problem therefore `is to transform this wide 4frequency television band into a bandV which maybe transmitted over the cable in the range between 100 kilocycles and 1000 kilocycles. This accomplished byrst 'modulating the wide frequency band uponfa carrierY current-of such' frequency that a considerable space in the frequency spectrum isleft between Vthe 4upper limit of. the original modulatingfrequency band, Vwhich will appear asal modulation product, and the lowermost frequencies of the lower side-band resulting from L the modulation. In selecting the frequency of -tion satisfactorily in a broad band television systhecarrier higher o'rder modulation components I have to be taken intofaccount. The next step in the process is that of lowering or shifting the transmitting television, Aspeech and television*` synchronizing current.` The particular range of Y frequencies; employed foreach transmission and their spacing in the :frequency spectrum were chosen, in accordance with the invention, ,as explainedbelow.v Let itV be` assumed that the coaxial cable is, designed to transmit television current of frequenciesfin the range 'between about 100 kilocycles andabout 1000 kilocycles Without undue distortion and cuts oif in the' region of -60 kilocycles. While a cable may be `designed to satisfactorily transmit lower tele-l vision frequencies, the` difficulties` encountered` are such as to increasethe cost. The cable when' digned to transmittelevison frequencies satisfactorily only above 100 kilocycles, introduces lphase shiftsat lowerfrequencies of such magnitude as to unduly distort the television image.`

It is also found-that in the regionV immediately below that where phase shifts for television be-` l come prohibitive, telephone transmission may be perfected with tolerable distortion and that a1- though theA synchronizing frequency must be transmittedwith a certain amount4 of fidelity the requirementsare 4not so stringent as to preclude the synchronizingffrequency or frequencies being transmitted in the region below 10,0 kilocycles. In accordance with this invention sound modulated carrier frequency tothe desired range for transmission, that is, arrangefrom 100 kilocycles to1000 kilocycles. This' is accomplished by selecting a single side-band and modulating it upona suitable'carrier. yIn the lutilization of a` single side-band it was desired toy employ these-called principle of vestigial side-band operation according to which one side-band is selected 'along with avestige of the other sideband in the immediate vicinity of the carrier frequencyV and the` selected l.side-band `is so shaped in the region adjacent thecarrier frequency corresponding in width Vto the vestigial Vside-band that at the receiver the vestigial sideband components may be made to combine with the complemental frequencies derived from the transmitted side-band in such a manneras to give the equivalenteffect of transmission onta single side-band having no' amplitude distortion in the regiorrof the carrier. This principle of providing a vestigial side-band `to enable single side-band transmission is disclosed in the pat- 'ent to H. Nyquist 1,748,186, patented February 25, 1930. The^problem here Vunder consideration', however, greatly complicates the application of the principleof vestigial side-band transmission in that inthe application of that principl'e great care must be used to design a lter the output of which has in it the complemental frequencies in proper phase as well as amplitude, y

and proper phase control at the frequencies here side-band principle.

kilocycles.

involved is most difficult. After selection of the wanted side-band and the vestigial other sideband the resulting current is applied to a second modulator along with a second carrier frequency of such value as to step the selected frequencies into the cable 'transmission range. There is employed a filter for the selection of the Wanted side-band and a vestige of the unwanted side-band which does not produce the amplitude and phase characteristics of the complemental frequencies of the two side-bands in a manner to make use of the operation disclosed in the Nyquist patent, thus avoiding the complication of having to design a filter or correcting network to obtain these proper amplitude and phase relations at the high frequencies which the filter must pass. The process of obtaining the correct amplitude and phase relation of the complemental frequencies is deferred until the band is stepped to a lowerrange in which adequate phase relationships as well as proper amplitude discrimination may be obtained. It will be seen however, thatthe band-pass filter after the first modulator in part accomplishes one step involved in the use of the vestigial side-band,

namely, the selection of the necessary frequen,

. its components or at a higher point where it would leave these components intact and cut olf the remainingportion of the upper side-band leaving a further undesired vestige. The latter course is adopted, the filter being designed to cut offA in such manner, taking into consideration the position' of the second carrier frequency,

lthat none of the modulation components of the y second modulator resulting from the unwanted vestigial will appear in the television transmission band range of 100 kilocycles to 1000 kilocycles. A band filter constructed to perform this Vfiltering operation introduces undesirable phase shifts which can, however, be corrected, along with subsequent phase shifts, after the second step of modulation, in order to obtain the proper phase relations'for application of the vestigial In the illustration. given a frequency of 2520 kilocycles was chosen for the second carrier so that the selected lower sideband appeared in the output of the second modulator as side-bands of the second carrier, the lowermost of which extended from about 70 kilocycles to 950 kilocycles, the position ofthe original carrier in the lower side-band at 144 kilo- Ycycles and the extremity of the wanted vestigial farthest remote from the carrier, at about 100 kilocycles. The corresponding upper side-band extended from about 4089 kilocycles to 4944 kilocycles. By means of a low-pass filter the lower side-band was then selected.

One requirement not yet mentioned was taken into account in the selection of the first and second carrier frequencies. It was desired to transmitthe speech in arange just below that suit-V able for television and the synchronizing frequencies in a range just below that suitable for speech. To accomplish this thepprogram and synchronizing frequencies were VV.modulated upon carriersxofl suitablefrequencyd place these transmissions in the.desired.rangesrespectively. The speech carrier, frequency was chosen at 84" After-(modulationv the lower side-A f band-extending to about73 kilocycles v was se'- lected for transmission. A frequency for the synchronizing carrier was then selected such that it would fall below and close to the speech band and still be suitable to serve as a base frequency for generating harmonic frequencies for the first and second television carriers. Taking into consideration all the requirements above stated it was found that these requirements could be met by selecting the synchronizing carrier at 72 kilocycles, impressing this frequency upon a harmonic generator to generate odd harmonies, and selecting the thirty-third harmonic for the first television carrier and the thirtyfifth harmonic'for the second television carrier.

This 'put the first television carrier at 2376 kilocycles, in a position where proper selection between the wanted and unwanted products of the first modulation could be made without introducing difficulties introduced by selecting the 4carrier at a higher point. It was found that the thirty-fifth harmonic, having a frequency of 2520 kilocycles fulfilled all the requirements lset forth above with respect to the choice of this carrier. The frequency in the lower side-band resulting from the second modulation which corresponds to the first carrier frequency of 2376 kilocycles and the zero component of the original television signal is 144 kilocycles, namely, the difference between the frequencies of the first and second carriers. This reference frequency of 144 kilocycles falls in the television range suitable for transmission overthe coaxial cable. The shifted frequencies,corresponding to the lower side-band of the 2376-kilocycle carrier modulation lie above this reference frequence of 144 kilocycles and the vestigial upper side-band selected by the band-pass filter from the first modulation products lie below this reference frequency.

A specially designed modulator-in accordance with this invention is used for the first modulator in order to effect complete suppressionv of the first carrier of 2376 kilocycles. This modulator comprises an active vacuum tube on the input of which the wide band of television frequencies are impressed and a passive vacuumV tube for suppressing the unmodulated carrier. A modulation component of carrier frequency corresponding to zero frequency components in the television signal does appear in the output of this modulator. While 'the invention is .being described in a single embodiment making use of a concentric cable as the transmitting means, it should be understood that in certain of its broader aspects the invention is not limited to v cable operation andthat some of the features fdescrib'ed. To illustrate the completesystem, Fig; ishouldjbe placed ron the left-hand side of Fig. v2 withthe lines `L' joined together. The circuit elements located at the transmitter are shown in the lower portion of Fig. 1 and those located at the receiver are shown in the lower V.portion of Fig;12. Thefrequency bands present at various points in the vcircuit and the energy distribution within these bands, sometimes spoken `of `herein as the shape of the band or the band shape, are shown-in the upper portions l of Figs. 1 and 2.

The television signa or video signal originates in a light sensitive electric deviceJ, such as a Y' photoelectric.4 cell or a photoelectric electron multiplier, energized by light under the control of suitable scanning devices. For the system `being described, a scanning disc` provided with Y a circle of lenseswas used to scan a uniformly moving sound picture film. Each frame was scanned in240: lines at the rate of `21 frames per second producing a video-signal extending from zero frequency to an upper useful frequency of i Yapproximately 806 kilocycles.

light energized a photoelectric electron multii plier to produce the video signals. i The major portion of the video signalvband is A,

shown by cross-hatched diagram 2.., 4The relative energy content ofV thevarious frequency components is shown by the horizontal distance between the vertical line` 3, which is a zero refer-` Y ence line, and the line 4. The video signal band,

Vas mentioned above, extends from zero frequency to approximately 806,kilocycles, as indicated by the logarithmic frequency scale on the left-hand side ofFig. 1. A corresponding scale appears on the right-hand side of Fig. 2. This video signal :is impressed on a rst modulator 5 to modulate A a carrier Vfrequency of 23'76v` kilocycles.V Modu- Q lator 5 is` specially designed to suppress practi-4 callyV completely all unmodulated c arrier current. The important frequencies appearing in --the output circuit of. modulator 5 are a band of frequencies corresponding to the video signal represented by cross-hatched .diagram 6 and up- ;perand lower side-bands on either side .ofthe y23'l6 kilocycleV carrier c represented by crosshatched diagrams 1 and 8. The lower side-band extends from approximately 1570 kilocycles up to the carrier frequency 2376 and the upper sideband 8 extends from this carrier frequency up to approximately 3182 kilocycles.

The frequency components in thetoutput of .modulator 5 are impressed on a band-pass filter 95 which suppresses the video signal band 6 and' alarge portion of the upper side-band 8 andV transmits the lower side-band 'I and a vestigial sideband represented respectively by the crosshatched diagrams 9 vand I0. Thecut-off of filter 95 in the neighborhood of the zsve-kiiocycle carrier isrepresented by line II extending to the leftof reference -line I2.A The vestigial side-band extends to approximately 2450 kilocycles.

This lower` side-band 9 and vestigial4 upper side-band I0 are impressed on a second modulator I3 to modulate a carrier frequency of 2520 kilocycles. The important frequencies appearing 1 inthe outputcrcuit of modulatorql3 are the upper andlowerside-bands corresponding to thel bands 9 and lI0 passed by the filter` 95 and the bands 9 vand I0. The portion of the lower sideband corresponding to band 9 is represented by cross-hatched diagram I4 and the portion corresponding to the.vestigial"sideband l0 is represented by cross-hatched diagram I5, these two `portions I4 and I5 lying, respectively, above and below 144 kilocycles, which :is the `component of' the lower side-band of the second modulator I3 corresponding to the first carrier of v2876 kilo- The scanning.

= grams I9 and 28, respectively. ,off of filter I8 is represented'by line 2IV extend- YingV to the left of zero reference line 22. These transmitted bands I9 and 20 extend from apcycles and the zero frequency component of the i video signal` 2. This lower side-band extends from approximately 'l0 kilocycles to 950 kilocycles. lThe portion of the upper side-band cor- .responding to band 9 `is represented'by crosshatched diagram I5 andthe portion corresponding to thevestigial side-band I8 is represented by cross-hatched diagram I1. This upper sidebandextends fromapproximately 4089 kilocycles to -4944 kilocyc1es Modulator I3 is a. balanced modulator which transmits the modulating bands V9 and I8` represented by cross-hatched diagram 96 but suppresses the carrier o`f 2520 kilocycles.

The 4frequency vcomponents in the output of second modulator I3 are impressed on a lowpass filter I8 which passes bands I4 and I5 and suppresses any 2520kilocycle second carrier leak,

the modulating band y96 and the upper side- The transmitted bands I4- bands I6 and I1. Y v and I5 are 'represented by cross-hatched dia- The upper Vcutproximately-'IO kilocycles to 950 kilocycles.

The transmitted bands I9 and 2Ilfare amplified in amplifier 23 and passed" through a line predistortion networkl 24,' another amplifier 25, an

` aperture equalizer 26`,"a .r iotherv amplifier 21 and a terminal equalizer 28. The combined attenuating 'characteristics ofnetwork 24 and equalizers 26 and 28 are represented bythe funcross-hatched diagram 29 and the shape offthetr'ansmitted components in the neighborhood ofi 144" kilocycles to effect approximately one-half the Ieffect required by the principles disclosed lin Nyquist Patent 1,748,186, supra. The frequencyV components corresponding to bands I9 vand'20 appear in the output of lter 3I` with relative energy l values or band shape, as represented by crosshatched diagrams 32 and 33. 4The lower cut-off of high-pass filter 3I is represented by line 34 extending to the left of zero reference line 35.

' These bands 32 and 33 are amplified in amplifier 36 and transmitted to the line FLwhich, preferably, is a coaxial cable provided with suitable amplifiers and equalizers to transmit a wide band of frequencies with `comparatively little distortion.

The band 33 above 144 range of the video lsignals and is a complete sideband, while theband 32 is a ve'stigial side-band.

Thefcomponent frequencies in the complete sideband, which `are complementary to thosev in the vestigial side-band, are attenuated about onehalf'v enough to compensate for the presence of vthe vestigial side-band. Filter 3l also adjusts the phase of' the'tra'nsmitted frequency components to compensate for phase distortions introduced by preceding circuit elements.

' Duetothe transmission characteristics of the coaxial cable and included amplifiers, the band shape of the transmitted bands v32 and 33 is changed somewhat during transmission over the coaxial cable.V `The Shape of these bands as they It isv seen from diagrams32 and 33 that there, is a band of transmitted frequencies on either side of 1 44,kilocycles. kilocycl'es corresponds toY the whole essential Lis represented bycross-hatched diagramv 31.

This band 31 is passed through high-pass` lil- Y appear at the receiver terminal of coaxial cable ter 38 and low-pass filter 39. These filters elimi- Y nate interfering frequencies outside of the transymitted band 31 and further shape the band in The shape the neighborhood of 144 kilocycles; of the band transmittedby lters 38 and 39 is represented by lcross-hatched diagram'l 40 comi prising the portion 4| above 144 kilocycles and the vestigia] side-band portion v42 below 144 kilocycles. The cut-ofi of high-pass'iilter 38 is represented by line 43 extending to the left-of zero reference line 44. The'cut-ofi of low-passrfilter 39 is represented by line 45, also extending to i'.

the left ofzero reference lineV 44. This band 40 extends from 100 kilocycles to approximately 950 kilocycles.

important frequencies appearing in the output of demodulator 63 are the video signal which is substantially identical with that impressed on the first'modulator 5 except that it is equalized to compensate for aperture distortion in the scanning devices, represented by the cross-hatched diagram 64, a band of frequencies corresponding to the modulating bands 60 andl represented by The frequencies in the video signal band 64 extransmitted band is represented by cross-hatched diagram 49 composed of complete side-,band 50 and a vestigial side-band 5 The band 49v comprising bands5ll and 5| is impressed on a first demodulator 52 to modulate a carrier frequency of 2520 kilocycles. This demodulator 52 is designed to suppress the 2520- kilocycle carrier. The important frequencies appearing in the output of demodulator 52 are an upper and lower side-band andv the modulating band 49 represented bycross-hatched diagram 91. The lower side-band may be considered as comprising a band extending below 2376 kilo-fY cycles and a vestigial side-bandv extendingabove 2376 kilocycles represented, respectively, by crosshatched diagrams 53 and 54. The bands 53 and 54 correspond, respectively, to the `portions 50 and 5|` of the modulating band 49; These two bands extend from approximately 1570 kilocycles to 2420 kilocycles. The upper side-bandis represented by cross-hatched diagram 55. 'I'his band extends from approximately 3470 kilocycles.

'I'he frequency components in the output of 2620 kilocycles to demodulator 52 are impressedon a band-.pass

filter 56, an amplifier 51 and a 2520-kilocycle band elimination filter 58. The band-pass filter 56 passes the bands 53 and 54 which make up the lower side-band of demodulator 52 represented, respectively, by cross-hatched diagrams 98 and 99, and suppresses the upper side-band' in accordance with lthe principles ofthe Nyquist patent, supra. AThe attenuation characteristic -of' the band elimination lter58 is represented by line 62. The nal shaping effected by this filter 58 is inherent in its attenuation characteristic and is compensated for by undershaping in highpass filter 38. Y

The finally shaped bandsjl) and 6| are impressed on a second demodulator 63 to modulate a carrier frequency of 27376 kilocycles applied in,

proper phase relation to the frequencies of the modulating bands 60 and 6| to produce a signal current according to the Nyquist principle. The

tend from zero frequency to about 806 kilocycles. The frequencies in band 65 extend from about 1570 kilocycles to 2420 kilocycles. The frequencies in the upper side-band 66 extend from approximately 3946 kilocycles to 4796 kilocycles. The second demodulator 63 is specially designed to suppress the carrier frequency of 2376 kilocycles completely for all practical purposes.

The frequency components in the outputI of second demodulator 63 are impressed on a lowpass filterwhich passes only the video signal band represented by cross-hatched diagram 68. This video signal band 68 is applied to cathode ray tube 69 to control the brightness of the scanning spot on the fiuorescent screen. This video signal band 68 has a form, suitable for compensating for the finite size of rspot in the cathode ray tube, this compensating shape resulting from the operation of the aperture equalizer 41.

The 2376-kilocycle and 2520-kilocyc1e carrier vfrequencies accurately spaced are derived from band-pass filter 13 and applied to the first modulator 5.V The thirty-fifth harmonic or 2520- kilocycle carrier is also selected from the output of harmonic generator 12, passed through crystal band-pass filter 14 and applied to the second modulator l 3.

A 5.76-kilocycle frequency for controlling the sweep circuits of the cathode ray tube 69 is obtained directly from a synchronizing impulse producer 15 associated with the scanning disc. This 5.76-kilocycle frequency from synchronizing impulse producer 15 is used to modulate a portion of the standard 72-kilocycle frequency from filter 1| in modulator 1'6. The 72-kilocycle carrier and the lower side-band frequency of 66.24 kilocycles resultingfrom the modulation lare selected by double band crystal filter |15, which frequencies are impressed on the coaxial cable L at the transmitting terminal and transmitted to the receiver,

At the receivingterminal the 72-kilocycle carrier and the 66.24-kilocycle side-band frequency are selected from the coaxial cable L by the double band crystal filter 11 and impressed on demodulator 18, from the output of which the 72- kilocycle frequency is selected by crystal bandpass filter 19, and the lower side-band frequency of 5.76 kilocycles is selected by the crystal bandpass filter 80. The 5.76-kilocycle frequency is used to control the sweep circuit generator 8| which furnishes the sweep currents for cathode ray tube 69. The 72-kilocycle frequency from filter 19 is impressed on harmonic generator 82. From this harmonic generator 82 the 2520-kilocycle carrier is selected by crystal bandcable. A y y As hereinbefore described the light signals coming from the scanning disc fall upon the r y p 2,273,548 pass fnl-.erts and applied to the nrstdemoduntor-52 and the 23'76-kilocycle carrier isselected by crystal band fllter84 and applied to the second demodulator 63S in properphase.` The phase .may be adjusted manually, the proper adjustmentbeing indicatedby the appearance of the l producedA image on the screen of the cathode ray tube |59.4 y

The'program frequency :band is transmitted as modulationsy of van` 84-ki1ocyclecarrier from y,carrier source 10. Theprogram frequency band is derived from the soundtrack on the nlm in sound `reproducer 85. y The 84-kilocycle program carrier frequency is passed throughz crystal bandf pass Vfilter 86 and, together with `the Yprogram modulator 92 by band-pass4 filter 93 and applied to loud-speaker 94 for program reproduction.

The relative positions of the program band,

the` 72-kilocycle standard frequency, andthe syn-l cnronizing 66.24-kilocycle side-band frequency with respect tothe transmitted television'band 31 `on thejcoaxial cable L are shown below the cross- ;hatched diagram31 infFig.- 2.' Two pilot chan- ,nelsare alsoprovided, as 4indicated in Fig. 2, at

the filterconsistingbfcondensers ||5Jand ||6 vand coils ||1-and ||8. The passive tube |09 serves thesole purpose of` balancingout the carrier frequency rinthe output of the modulator 5. Condensers ||9l and |20 and resistanceV |2| are adjusted to make the impedance facing the control grid of the passive tube |09y equal to that facing the control grid of the active tube |08 at carrier` frequencies. The negative control grid bias of the passive tube |09 is furnished by battery |22 and may be adjusted by means of potentiometer |23 to make the carrier frequency trans-` mission of the passive tube |09 equal tothat of the active tube |08. The negative bias onA the control grid of the active tube |08 is furnishedby battery |24. The positive screen bias is furnished by the tapon battery- |25,throughco il |26 and .the screens `are shunted by'condenser |21. The two tplatesl are shunted by resistances |28 and |29 and connected to; an output network consisting `of condensers |30 and|3| and transformer |32. Condenser |30really consists of two Condensers, one of which subtracts capacity from one plate circuitas it is added to the other.- The law of variationgoverning tliese two condensr'sis'fsich that the total` vseries .capacitativev impedance across both plates is at all times kept constant.

Coil. |33 4andcondenser |34 form a circuit tuned to the second harmonic of the 2376-kilocycle car-l l rier` frequency to aid in its suppression. It will beseen that there `are two controls affecting the balance of .carrier frequency at the output of the I rst modulator, namely, the potentiometer |23 `1'024 kilocycles and v60 kilocycles for controlling the gains of the coaxial cable amplifiers.

Referring now to- Figs. 3 andl, the circuits of the first modulator 5, band-pass filter 95,' second modulator .|31' and theconnection to the first Vmodulator 5 of the light sensitive electric device in-the form of aphotoe'lectric electronmultiplier 'will lbe described. The fcoaxial cable-|1| at the riglfitside vof Fig.3 andthe vcoaxialcable V|12 at th'eleft side of-fliiig. fl' are portions of the'same vphotoelectric .cathode of the electron multiplier A| 0 which transforms them into r video signals whichl are transmitted fromv the multiplier |0| by'means of a section of doubleshielde'd coaxial Cable |02 to the rst modulator 5. In the input circuitof the first .modulator 5 thefcondensers |03, |04 and |05, and `coil |06 form a `low-pass network which when terminatedin theresistance '|01 nominally 1,000-l ohms present a1000-ohm whichvaries the gain ofthe passive tube |09 and the condenser |30.which act as a phase balance in the output circuits." -By means of this double adjustment it is possibleto suppress the un:

wanted carrierfrequencyhin'the output .of the modulator by` approximately 80 decibels. `The 'carrier supplied to this modulator 5 modulates the videosignal coming from'the electron multi` plier |0| and its output contains twovcomplete side-bands. 1t.is to be noted that since the input circuits `toy the modulatortransmit all the way down todirect current, any direct current coming from the electron `multiplier. will una balance the modulatorc'ausin'g immodulat'ed car` rier frequency to flow in itsvoutput which is exactly proportional to the direct current in the inputcircuit. 'This makes` it possible forinformation concerning the average background of the picture to 'be'ftransmittedl 'The band filterv|35 selects the lower side-band `of the carrier and a Asmall portion of'the upper side-band. The input network of` the second modulator |3 consisting of transformer |36, Condensers |31`and |38 and resistances |39 and |40is similar to the output network of the `first modulator v5. The second impedance to the terminalsof the electron multil plier atvvideo` frequencies. "The vacuumytubes |08 and, |09 in the first modulator 5 are so-called spacel charge pentodes.r After the cathode the rst electrode is a space charge grid, hereafter known as the net electrode. The next electrode is the ordinary control grid, the third a screen grid, and the last the plate. The video signalis 'placed on the controlv grid. The 2376-kilocycle rcarrier frequency comes in on a-coaxial cable ||0` to a potentiometer and is appliedvacross'the net electrodesl of bothtubes |08 and |09. This carrier circuit is bridged by a resistance ||2 in series with a thermocouple I|3 for the purpose of reading the carrier frequency voltage applied.

The positive direct current voltage on the net electrodes is supplied by the battery I4 through modulator |3 is a kconventional balancedmodulator employing power pentodes |16 and |11.

They signal frequency is` applied to the control s grids of the, two tubesin series and the carrier frequency isapplied to the control grids of the twotubes in parallel. 'The 2520-kilocycle carrier frequency is'applied through a parallel tuned circuit consisting of condenser |4| and coil |42 across the resistance |43. Potentiometer |44 allows a balance to be obtained against the carrier frequency of the second modulator I3 in its output circuit. Plate and screen potentials for the tubes are obtained from a 250-volt supply and applied to the screens through coil |45 and to the plates through coil |46. Condensers |41 and |48 in conjunction with coils |45 and |46 act as power filters for supplying the screen and has a capacitance of 184.9 micro-microfarads less the inherent capacitance of the section of coaxial cable |02 and the electron multiplier |0|. The condenser |04 has a capacitance of 73.95 micromicrofarads. Condenser |05 has a capacitanceof 40.19 micro-microfarads less the inherent capacitance of active" tube 8. Coil |06 has an inductance of 125.8'microhenries. Resistance |01 has a value of 1,000 ohms. Vacuum tubes |08 and |09 are identical and preferably are Western Electric K code No. 7575F vacuum tubes. Potentiometer I has a resistance of 400 ohms and resistance ||2 has a value of 1,800 ohms. The thermocouple I3 has a resistance of 500 ohms. Battery ||4 is of 10.5 volts'. Condensers ||5 and ||6 each have a capacitance of .03 microfarad. Coils ||1 and H8 each have an inductance of lemillihenry.

Condenser ||9 is adjustable'and has a nominalA capacitance of 50 micro-microfarads. Condenser |20 has a capacitance of 20 micro-microfarads. Resistance |2| has avalue of 1,000 ohms. Battery |22 is of 9 volts. Battery |24 is of 1.5 volts. The total voltage of battery |25 is of250 volts and the lower portion of 160 volts. Coil |26 has an inductance of 1 millihenry. Condenser |21 has a capacitance of .'7 microfarad. Resistances |28 and |29 each have a value 4of 3,000 ohms.

`The left-hand windingof transformer |32 has an inductance of 243 microhenries and right-hand windings in parallel have an inductance of 20.2 microhenries. Condensers |30 and |3| are adjustable and in parallel with the tube capacities are set to .tune the left side windings of transformer |32 to 1,862 kilocycles. y Coil |33n has an inductance of 100 microhenries andcondenser |34 has a capacitance of 25 micro-microfarads, the condenser |34 being adjusted to form a tuned circuit with coil ||3 tuned toa frequency of 4,752 kilocycles.

Referring now to Fig. 4, the band filter |35 comprises two series elements in the form of parallel tuned circuits |60 and |6| and a shunt element in the form of a parallel tuned circuit |62. Tuned circuit |60 consists of a coil |63 having an inductance of 149.6 microhenries and a condenser |64 set to tune this circuit |60 to a frequencyof 1,000 kilocycles. Tuned circuit |6| consists of a coil |65 having an inductance of 21.0 microhenries and a condenser |66 set to tune this circuit |6| to a frequency of 2,625 kilocycles.

Tuned circuit |62 consists of a coil |61 having any inductance of 55.3 microhenries and a condensery |66 set to tune circuit |62 to a frequency of `2,350 kilocycles. Condenser |69 has a capacitance of 344.8 micro-microfarads and condenser a capacitance of 195 micro-microfarads.

Still referring to Fig. 4, the left-hand winding of transformer |36 has an inductance of 47 microhenries and the right-hand winding has an inductance of 278 microhenries, each of the t'wo condensers making up condenser |31 have a capacitance of 10 micro-microfarads. Condensers |31 and |38 in parallel with the inherent tube capacities, are set to form a tuned circuit with the right-hand winding of transformer |36 tuned to a frequency of 1,790 kilocycles. Resistances |39 and |40 each have a value of 3,000 ohms. Condenser |4| and inductance coil |42 are adjusted to be antiresonant at 2,520kilocycles. Resistance |43 has a value of 100 ohms.A Potentiometer |44 has a resistance of 150.0hms while the Condensers connected to either terminal of resistance |44 each has a capacitance of .7 microfarad and the resistance connected to the variable contact of resistance |44 has a value of'500 ohms. Each of resistances |50 and |5| has a valueof 2,140 ohms. Resistance |52 has va resistance of 260ohms. Tubes |16 and |11 are identical and preferably are Western Electric code No. 7629 vacuum tubes.

The high-pass filters 3| and 38 are constructed each with amplitude attenuating sections and phase adjusting sections according to wellknown filter design principles. Sufcient sec' tions of each kind are used to produce the band shapes described hereinbefore.

What is claimed is: 1. A modulator for a broad band of frequencies comprising an active tube and a passive tube each having a cathode, a net electrode,- a control grid, a screen grid and a plate, means to produce a broad band of signaling frequencies, means to impress said signaling frequencies on the cathode and control grid of said active tube only, means to impress a carrier wave on the cathodes and net electrodes of both said tubes in parallel, means to impress direct current biasing potentials on the control grids of Yboth said tubes, and means to independently adjust the biasing potential on the control grid of vsaid passive tube and the alternating current .impedances between the cathodes and the plates of both said tubes to completely suppress any component of unmodulated carrier current in the output circuit of said modulator.

2. A modulating Ysystem of the suppressed carrier type comprising a pair of electric discharge tubes, each including a cathode,v an anode, a control grid and a 4second vgrid between said cathode and control grid and an output circuit,

means for impressing negative biasing potentials on said controll grids independently, means for impressing modulating currents on one of said control grids only, means to impress carrier current on said second gridsI in like phase relation, means to impress positive biasing potentials on said second grids,V and means coupling the output circuits of said tubes together and to a work circuit to suppress the unmodulated component of carrier current in said work circuit.

CHARLES L. WEIS, JR. 

